I use SB400 when I can and the SB600 when it's required. I can get 2 SB600 for the price of one SB900 which I would prefer.

The SB400 recycles faster than any of them and is great for daylight photography...fill. I angles only up, but most of the time that's plenty for me.

It really depends on what you want to do with it. I like to lug as little stuff around as I can, so the small form factor of the SB400 is perfect for me. But if you must have more power, larger ones are more suitable.

Depends on your budget, the SB-600 is a very capable flash, however the SB-700 is even better, the SB-700 is also much more expensive. If you can afford it, go for the SB-700 for sure, otherwise the SB-600 is still a great choice.

I personally wouldn't recommend the SB-400, I really think that it's just a slightly improved pop-up flash. For an external flash, the power isn't superb, and it's only real advantage is that you can bounce light. It is portable though.